1. Field of the Invention
This invention relates to a method for manufacturing heat-treated, biaxially oriented, thermally stable, blown containers suitable for hot-filling applications, and apparatus for manufacturing the same.
2. Description of the Relevant Art
In recent years, plastic, especially polyethylene terephthalate (PET), has been more and more widely used to make beverage (and food) containers, as it is inexpensive and durable. However, in the process of bottling many beverages, the plastic containers must be filled with the beverage at an elevated temperature sufficient to ensure sterilization. Exposure of the plastic container to the hot beverage can cause the plastic to soften and deform, especially over time, because the temperature of the beverage is often above the glass transition temperature of the plastic. Moreover, because such containers are typically made by blow-molding a preform, there is a large amount of stress frozen in the container walls. This stress, when it relaxes in response to heat, causes the container to shrink.
Beverages which are pasteurized, as is the case with many European drinks, are treated in a range of 148.degree. to 170.degree. F. Drinks which include a portion of fruit juice are typically "hot-filled" in a range of 170.degree. to 185.degree. F. In these ranges, two currently-used single-mold processes are that of U.S. Pat. No. 4,863,046 to Collette et al and assigned to Continental PET Technologies, Inc., and another process involving blow-molding in a heated mold and maintaining the article in contact with the heated mold walls, such as described in U.K. published patent specification No. 1,474,044 by Collins and assigned to Imperial Chemical Industries Limited.
The Collette et al process involves a series of heat treatments to a preform, followed by axially stretching the preform, then blow-molding it in a heated mold. The Collins process involves blowing the preform in a hot mold, usually around 100.degree. C., and maintaining the resultant container in contact with the mold walls for a period of time, such as 6 to 8 seconds or longer. For example, Nissei ASB Machine Co., Ltd. practices a variation of this method in which a preform is blow-molded into a hot mold and maintained in contact with the mold walls for up to about 25 seconds or longer as allowed by the longest other process in the manufacturing cycle. The hot mold walls will enable some limited thermally-induced crystallization and some limited annealing, i.e., limited release of stresses created by blow-molding. The mold walls can be heated up to temperatures which are much higher than 100.degree. C., but, at some point, the plastic container will begin to lose its rigidity and utility, depending on the material used, its thickness, the contact time, and other factors.
These single-mold processes achieve moderate thermal stability. However, at higher hot-fill temperatures, i.e., 190.degree. F. and up, which are needed to sterilize fruit drinks and the like, these single-mold processes are not sufficient to enable the resultant container to withstand such hot-filling without substantial deformation. The combination of these elevated filling temperatures and the time for which the container walls are exposed to these temperatures during filling, which can be as much as two minutes or more, increases the mobility of polymer chains and allows stresses to relax that would not normally relax during either of the above single-mold processes. This is because the PET polymer has a range of molecular weights, and the molecules accordingly take a range of times to relax. That is, some of the PET polymer molecules will relax quickly, while other PET polymer molecules will take longer times to relax. Because of the limited temperatures at which the mold walls can practically be kept, and the limited time at which the container can be kept in contact with the mold walls due to manufacturing production constraints, these single-mold processes cannot achieve sufficient relaxation of stresses to enable the resultant container to withstand this higher range of hot-filling temperatures. Moreover, due to polymer "initialization" times, i.e., the threshold time at a given temperature that it takes a polymer before it will begin to relax, and due to the practical limitations of the manufacturing process upon the mold wall temperature and contact time, there is insufficient heat and time to obtain any substantial relaxation of the "longer-term" stresses. Therefore, to provide improved shrinkage resistance, two-mold blow-molding processes in which a first blow-molded article is heated to a relatively high temperature, or for a relatively long time, and allowed to shrink as a result of the heating, then is reblown, have been proposed.
One such two-step process is disclosed in U.S. Pat. No. 4,550,007 to Ohtsu et al and assigned to Mitsubishi Plastics Industries Limited. In this patent, the first molded article can be slightly larger than the desired final container. The first article is heated in the first mold by keeping it in contact with very hot mold walls, e.g., from 130.degree. C. for 5 seconds to 230.degree. C. for 2 seconds. Then the article is removed and allowed to freely shrink. It is promptly transferred to a second mold heated at 60.degree. to 150.degree. C. where it is blown into a final container
In European patent application No. 0,155,763, assigned to Yoshino, a similar process to that of Ohtsu et al is disclosed. That is, the first article can be slightly oversized in relation to the final desired container, and it is heated in the first mold, then allowed to freely shrink and then reblown.
In both of these two-mold processes, the initial blown article is heated in the mold and then allowed to freely shrink. The free shrinkage of the first article creates unevenness in its wall thickness and thus unevenness in the final blown container. Moreover, the time that the article remains in the first mold for heating is limited by practical constraints upon the molding process. The longer the heating, the longer the molding process, and the less commercially attractive due to lower production rates.
Another two-blow-molding-step process is disclosed in U.S. Pat. No. 4,836,971, issued to Denis et al and assigned to Sidel. In this process, the first blown article is heated in a large oven, separate from the first mold. The process starts with a pre-made preform which first has its neck heated to crystallize it, and then the neck is cooled. The preform is then heated, using infrared heaters, to an appropriate temperature between 100.degree. to 120.degree. C. for 10 to 80 seconds while being rotated, and then subjected to blow-molding in a cold first mold, i.e., 5.degree. to 40.degree. C. which does not provide any appreciable stress relaxation. The first mold cavity is larger than the final desired container, such that the first blow-molded article is larger than the final container. This oversized article is then heated in the oven at 180.degree. to 220.degree. C. for 1 to 15 minutes and allowed to freely shrink. The crystalline neck is needed to withstand the heat of the oven so that the neck does not deform. The article shrinks to a size smaller than the final desired container and experiences uneven shrinkage such that it typically has a chili-pepper-like shape. The shrunken article is then placed in a final mold and blown to form the final container.
This method often results in a final container having a crystalline haze and an opaque white crystalline base, both of which are undesirable. This haze occurs due to the unrestrained free shrinkage of the article which allows the development of zones of crystallinity on a macro-scale. That is, such zones of crystallinity exceed the wavelength of light in a size, and this causes the milky crystalline haze. Moreover, because shrinkage is unrestrained, there are areas of weakness created in the shrunken article such that, upon reblowing, there can be pinholes or weakened areas in the final container causing it to be rejected. In addition, the center of the bottom of the article can wander, causing the final container to be lopsided. Further, the distribution of wall thickness is nonuniform.
It is also necessary in this Denis et al method for a separate machine to make preforms from the first blow-molding machine. In addition, because the preforms are pre-made, there must be equipment to heat them to an appropriate temperature for stretching in the first blow mold.
In addition to the heat of hot-filling causing shrinkage and a reduction in container sidewall stiffness, there is also an internal vacuum created in the container as the hot-filled liquid cools. Therefore, buckling of the container can occur, especially if the sidewalls have weakened areas.
Accordingly, there is a need for a process to make a plastic, e.g., PET, container that is stable, even under classic hot-filling conditions, e.g., above 185.degree. F., and especially 190.degree. to 203.degree. F. and above, and sparkling glass-like containers.